In general, various processes such as a film forming process, a quality modification process, an oxidation/diffusion process, an etching process, and the like are performed on a target object such as a semiconductor wafer to manufacture an integrated circuit of a semiconductor product. Recently, with a demand for further increasing the density of the semiconductor integrated circuit while decreasing the film thickness thereof and a demand for a higher level of miniaturization of the semiconductor integrated circuit and a need to process it at a lower temperature, a processing apparatus using plasma tends to be widely employed. In the processing apparatus using the plasma, active species are generated by activating a gas by the plasma, and a desired process can be carried out by the action of the active species in a relatively low temperature state, without having to heat a wafer up to a high temperature. Such plasma processing apparatus using the plasma includes a high frequency wave generation device for generating a high frequency wave or a microwave generation device for generating a microwave, both being employed for the creation of the plasma.
Here, description will be provided for the case of using the microwave generation device, for example. The microwave generation device includes a magnetron configured as a bipolar tube wherein an anode having a cavity resonator is coaxially disposed around a cathode having a filament. In the device, by oscillating a high frequency wave while imposing a DC magnetic field between the two electrodes in an axial direction, a microwave is generated (see, for example, Patent References 1-3).
Specifically, in this magnetron tube, by supplying an electric power to the filament, the cathode is heated so that thermal electrons are released therefrom, as in an ordinary bipolar vacuum tube. At this time, electric current is controlled by an electric field (voltage) applied between the two electrodes. At the same time, by means of creating rotational motion of the thermal electrons by a magnetic field formed in a direction perpendicular to the electric field, an oscillation phenomenon occurs, whereby the microwave is generated.
[Patent Reference 1]
Japanese Patent Laid-open Application No. H5-67493
[Patent Reference 2]
Japanese Patent Laid-open Application No. H10-223150
[Patent Reference 3]
Japanese Patent Laid-open Application No. 2003-308958
The filament of the magnetron is generally made of a tungsten metal material containing a small amount of thorium, and a carbonized layer is formed on the surface of the cathode including the filament to facilitate the release of the thermal electrons. Expiration of the service life of the magnetron is resulted from excessive reduction of this carbonized layer.
A conventional magnetron control method has been as follows. In the event that an output of the magnetron is varied, a voltage applied to the filament (filament voltage) is varied in inverse proportion thereto to lengthen the service life of the filament, whereby an excessive temperature rise of the filament is prevented. For example, if an anode current is increased to enhance the output of the magnetron, the temperature of the filament would be increased as well, resulting in excessive consumption of the carbonized layer. For the reason, when increasing the anode current to enhance the output of the magnetron, the filament voltage is reduced to prevent the temperature rise of the filament, whereby the consumption of the carbonized layer due to the excessive temperature rise is prevented. On the contrary, when the anode current is reduced to decrease the output of the magnetron, the filament voltage is increased.
In the conventional magnetron control method, however, in spite of performing the control of increasing or decreasing the filament voltage in response to the increase or decrease in the output of the magnetron in inverse proportion in order to prevent overheating of the filament, the filament would actually suffer from a considerable variation of its temperature considerably. In some occasion, the filament would be overheated, and the carbonized layer would be consumed excessively, resulting in a great reduction of the service life of the magnetron.